DIVERSITY OF BASAL FOREBRAIN SOMATOSTATIN-EXPRESSING NEURONS
Péter Papp1, Zsuzsanna Hajós1, Krisztián Zichó1, Katalin E. Sós1, Zsuzsanna Bardóczi1, Márton Mayer1, Gábor Nyiri1
The basal forebrain regulates cortical activity, for instance ascending cholinergic, glutamatergic, and parvalbumin-expressing GABAergic cells have been shown to promote arousal, attention and wakefulness. However, at least one population of somatostatin-expressing GABAergic neurons is known to promote sleep through inhibiting cortically-projecting neurons. Here, using virus-mediated neuronal track-tracing techniques and immunohistochemistry, we investigated the projection patterns of the somatostatin-expressing (SOM) neurons of the basal forebrain. SOM cells were localized both to the caudo-ventral (horizontal limb of the diagonal band of Broca, substantia innominata, lateral preoptic nucleus) and rostro-dorsal (medial septum, vertical limb of the diagonal band of Broca) sub-regions of the basal forebrain. Although previous studies emphasized the role of SOM neurons in local inhibition of major ascending excitatory pathways, we found that some of them target inhibitory neurons as well. We also found that basal forebrain SOM neurons project to several cortical and subcortical areas, including the olfactory bulb, the medial and lateral habenular nuclei, the cingulate and dorsolateral entorhinal cortices as well as the hippocampal formation. Analysis of these pathways suggest that the basal forebrain harbors several different sub-populations of SOM neurons, which probably have different behavioral functions.